Surgical removal of epileptogenic brain is indicated for the treatment of many medically refractory focal seizure disorders. Such surgery demands a high degree of accuracy in identifying the epileptogenic foci. Various methods have been used in attempting to determine the location of these foci, and all involve sensing cortical electrical activity using electrical contacts applied in various ways.
While scalp contacts were customarily used for many years to identify epileptogenic foci, accurate localization of the loci was usually very difficult with the recordings obtained from such contacts. Therefore, many medical centers in recent years have progressed to using intracranial recording techniques to better define regions of cortical epileptogenicity whereby the safety and effectiveness of epileptogenic brain removal is enhanced.
Intracranial recording techniques have typically involved one of two different types of sensing devices—intracortical depth electrodes or cortical strip electrodes. While depth electrodes are necessary in certain circumstances, techniques using cortical strip electrodes have been shown to be relatively safe and serve as valuable alternatives.
The relative safety of cortical strip electrodes lies in the fact that, unlike depth electrodes, they are not invasive of brain tissue. Depth electrodes are narrow, typically cylindrical dielectric structures with contact bands spaced along their lengths. Such electrodes are inserted into the brain in order to establish good electrical contact with different portions within the brain. Cortical strip electrodes, on the other hand, are flat strips that support contacts spaced along their lengths. Such strip electrodes are inserted between the dura and the brain, along the surface of and in contact with the brain, but not within the brain.
A cortical strip electrode has a flexible dielectric strip within which a plurality of spaced aligned flat contacts and their lead wires are enclosed and supported in place between front and back layers of the material forming the dielectric strip. Each flat contact has a face or main contact surface which is exposed by an opening in the front layer of the dielectric strip. Insulated lead wires, one for each contact, are secured within the strip and exit the strip from a proximal end. The dielectric material used in such cortical strip electrodes is typically a flexible, bio-compatible material such as silicone.
While the typical cortical strip electrode works fine in many situations, there are applications for which its structure is not well suited. For instance, monitoring may be desired at a variety of positions around the surface of the brain. The placement of a number of strip electrodes, with their associated multiple contacts, may be more than is necessary. Cortical sensing devices that allow sensing elements such as electrical contacts to be individually positioned at various positions around the brain surface in an easy and safe manner would be an improvement over the current state of the art.
One can appreciate that when a sensing element of a cortical sensing device is placed in contact with the cortex, it is critical that the sensing element remain in that same fixed position relative to the cortex since knowledge of its exact position is necessary to properly interpret the device's readings. The typical cortical electrode is, however, not well anchored or held in place at its desired position upon the brain without being sandwiched between the dura and the cerebral cortex. With these electrodes, it is hoped then that they will not be moved and, for this reason, precautions are often made so as not to disturb the externally positioned lead wires. Nevertheless, movement of the electrode can occur and this movement may even cause inadvertent penetration of the brain. Thus, there is a need for an improved cortical sensing device which can be anchored at a desired position such that it is much less likely to penetrate brain tissue inadvertently and better able to remain at its selected position upon the brain.